It is in the Book of Inheritance (Kitab al-Fara'id) that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) wrote to Sayyiduna Abu Ubaydah (may Allah be pleased with him): Teach your boys swimming and your fighters archery. They were shooting at different targets when a stray arrow came and struck a boy, and he was killed.
(b) Sayyiduna Abu Umamah bin Sahl bin Hunayf (may Allah be pleased with him) said that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) wrote to Sayyiduna Abu Ubaydah (may Allah be pleased with him), and he mentioned it in the same way.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19739